Liberty City has one major road tunnel system, dubbed Porter Tunnel, possibly based on the Holland Tunnel, Lincoln Tunnel or the Big Dig, located in Boston. The Porter Tunnel is a primarily four lane underground and underwater road tunnel (the Staunton Island tunnel exit consists of only one lane for each direction), which connects all boroughs in Liberty City: Portland via Harwood, Staunton via Rockford, and Shoreside Vale via Francis International and Wichita Gardens. The tunnel was initially inaccessible, citing that tunnel construction was still incomplete and had been delayed several times. The tunnel would then be opened in phases: first with the segment linking Portland and Staunton, then the segment linking the Portland-Staunton exits to the Shoreside Vale exits.
In Shoreside Vale, entrances to two additional road tunnels may also be found north of Cedar Grove and Cochrane Dam, respectively. The tunnels were inaccessible in GTA III, but in Liberty City Stories, it is revealed that the tunnels can be accessed and connects to each other at an underground T-junction. The tunnel that branches off is signposted as leading to "Upstate" (presumably Carcer City), but is inaccessible.